1. Field of the Invention
The present invention relates to apparatus for reproducing a video signal and, more particularly, to a servo circuit of a VTR of the helical scan type.
2. Description of the Prior Art
As a tracking control system for use in a helical scan type VTR, a number of different systems are known. In one such system, a control signal or CTL signal is recorded on a track in the longitudinal direction of a magnetic tape. In another such system, four pilot signals having predetermined frequency differences with respect to one another are recorded on the tape. However, in all such conventional systems, the circuitry is quite complicated. This causes manufacturing costs to be higher than is desirable. For example, in systems using a CTL signal, a fixed head for recording and reproducing the CTL signal is necessary. This not only represents additional cost, it also reduces the recording density that can be achieved on the tape, since a significant amount of tape must be devoted to the CTL signal.
To solve such problems, there have been proposed tracking control systems in which the recording of a special signal for purposes of tracking control becomes unnecessary. Such tracking control systems are based on special processing of the reproduced RF signal. A so-called dynamic tracking system is representative of tracking control systems based on special processing of the reproduced RF signal. However, even in a dynamic tracking system, a voltage-dependent mechanical transducer such as a piezoelectric bimorph and a drive circuit are needed. Thus the construction is complicated and the manufacturing costs are inordinately increased.
Japanese patent application No. 63-241366, assigned to the assignee of the present invention, discloses a magnetic recording and reproducing apparatus in which tracking control is carried out in response to the level of the reproduced RF signal plus information relating to the accelerating or decelerating state of the magnetic tape.
However, in the tracking control system as disclosed in that patent application, no compensation is made for the nonlinear characteristics of the level of reproduced RF signal as a function of track deviation (i.e., as a function of deviation between the location of the reproducing head and the location of the previously recorded track). Thus even that system is not ideally suited to achieving stable tracking control.
FIG. 1 illustrates the nonlinear characteristics of the reproduced RF signal level as a function of track deviation. In FIG. 1, assuming that the reproduced RF signal level corresponding to a predetermined track deviation is required to be amplified, the gain must be larger at point 50A than, for example, at point 50B.
A drive amplifier to drive a motor as one of the elements for setting the gain of a servo system is provided in a servo loop. Since the gain corresponding to point 50A is large, the gain of the drive amplifier can be relatively small. On the other hand, since the gain corresponding to point 50B is small, the gain of the drive amplifier must be relatively large.
Therefore, if the gain of the drive amplifier is matched with the level at point 50A, the gain of the servo system may be so small at point 50B that an off-track indication occurs. On the other hand, if the gain of the drive amplifier is matched with the level at point 50B, the gain of the servo system may be so large at point 50A that an oscillation occurs. The servo characteristics thus deteriorate, and it is difficult to carry out the tracking control in a manner that is stable and reliable.